Ultrasound color flow imaging is a widely used modality which enables the clinician to view both venous and arterial blood flows. Color flow images are produced by mapping a color that corresponds to a sensed velocity onto the ultrasound image. When the user of the ultrasound imaging system alters the focal depth of the ultrasound beam to change the position of the focal plane of the ultrasound beam, current state of the art ultrasound systems do not alter the frequency of the transmitted ultrasound signal to assure a maximum backscatter signal. This is because a frequency alteration can change the mapping of flow velocities to color and result in changes in the color presentation, notwithstanding the fact that the imaged flow velocities remain relatively unchanged.
Certain facts are well known about ultrasound system performance;
1. The amount of energy which reflects off tissue and blood increases as the incident ultrasound frequency is increased. Further, the rate of increase of blood backscatter is larger than the rate of increase of tissue backscatter. PA1 2. The ultrasound beam is attenuated as it passes through the body. As beam frequency is increased, the attenuation also increases. Therefore, the amount of energy which actually reaches a given point in the body decreases with an increase in frequency. PA1 3. Known color flow imaging techniques which utilize pulsed Doppler and autocorrelation techniques take into account the pulse repetition frequency (PRF) and the ultrasound transmission frequency in order to estimate blood velocities.
The relationship between blood velocity, PRF and transmission frequency is given as follows: EQU Velocity.apprxeq.PRF/transmission frequency
Depending upon the particular portion of a body to be imaged, the user selects a transducer which is particularly designed to handle the frequencies that are utilized to image the particular body portion. The ultrasound system maintains a table which lists the transmission frequencies for the various transducers. Thus, when a particular transducer is connected to the system, a transmission frequency is determined from the table and is used thereafter to control the transducer. Such frequency assignment enables acquisition of optimum images of the body region of interest.
Upon a change of depth of focus by the user, the ultrasound system automatically alters the frequency applied to the transducer, in accordance with a predetermined mapping of image depth to frequency value. However, prior art systems do not concurrently alter the PRF and the result is an alteration of the color presentation, due to fact that the calculated velocity value is dependent upon both applied frequency and PRF. Accordingly, the velocity color attribute changes even though there is no change in the imaged velocity. This renders it more difficult for the user to assess blood velocities at different depths within the body.
Accordingly, it is desirable to provide a color flow Doppler imaging system wherein the color presentation of flow velocities remain constant, notwithstanding alterations in the depth of the body region being imaged.